1. The Field of the Invention
The present invention concerns a method and apparatus for determining the pitch of a wellhead and in particular to an underwater wellhead.
2. Prior Art
In cases where wells are drilled underwater and hydrocarbons deposits are discovered, it may be desirable to temporarily abandon the well such that it can later be "tied back" to a producing structure (platform). This allows the exploratory well to be used as an actual producing well; thereby, eliminating the costly redrilling of another well from the producing structure.
When planning and designing for these tieback operations, it is extremely critical to know the misalignment offsets that exists. One of these offsets is the angular misalignment of the wellhead as it exists underwater, near the mudline. This angular misalignment is the inclination of the wellhead as compared to true vertical reference. Not only is the angle of the wellhead (to within a quarter-degree) critical, but equally important is the heading or direction of the deflection. equally important is the heading or direction of the deflection. Often the water depth deters using divers to perform this reading, so it is desirable to obtain this reading using tools which are run from a vessel or structure above the waterline.
Previously there were two means which could be used to give an estimate of this angular misalignment.
a). One method involved measuring the angle found on the 0-5 degree bubble level indicator attached to the guidance and alignment structure (which had been attached to the jetting string). It had to be assumed that this reading would be equivalent to the inclination of the wellhead which was run on the subsequent casing string.
The shortfall of this system is that there is no guarantee that the inclination of an outer string of pipe would be the same as the inclination of an inner string which did not use centralization. Field results have shown that this assumption can misrepresent the wellhead angular offsets by as much as 150%. Another drawback is that the bubble indicator run on the guidance and alignment structure measures from 0 to 5 degrees with increment lines marked only in one degree intervals. This makes taking the reading to within a quarter-degree substantially impossible.
b). The other method involved running a gyroscope multishot survey tool through a flexible drillstring which would land on top of the wellhead. The surveying tool would land in a slotted profile that would allow an inclination measurement of the wellhead to be taken. If properly taken, this angular offset reading would be within the desired precision tolerances and would also give the heading of the offset.
The shortfall of this system is that the effects of wave and current forces acting on the bottom of this flexible drillstring cause fluctuations which prevent the drillstring from having continuous contact with the top of the wellhead. As a result, numerous survey points were needed and an approximate reading had to be determined using statistical analysis.